I may be dim, but I thought there were two (easy) point at which to fill up the tracta/bevel boxes?
the filler/level plug on the inner tracta/ bevel (page 131 of the tech manual), and the filler/level plug on the outer tracta ( why bother trying to get to the filler plug at the top)? It also says in the tech manual to fill the hubs at the outer tracta filler, with the hub plug at 3 or 9 o'clock until it flows, replace plug and continue to fill until it flows from outer tracta filler/level plug.
i have always filled mine from the easiest position (ie the filler/level plugs), taking it slow and rechecking as you do tend to get a little bit of blow back and time to settle.